1. Field of the Invention
The present invention relates to a wet multi-plate clutch used in a clutch and/or a brake of an automatic transmission (AT) of a vehicle.
2. Related Background Art
Conventional wet multi-plate clutches used in automatic transmissions generally comprise separator plates as external toothed plates and friction plates as internal toothed plates constructed by sticking friction materials to core plates, and are designed to transmit torque by engaging the external toothed plates with the internal toothed plates.
In the past, as shown in Japanese Patent Application Laid-open No. 11-141570 (1999), a wet multi-plate clutch constituted by sticking friction materials to both surfaces of friction plates as internal toothed plates is known. Further, a clutch design of one-surface alternate sticking type in which friction materials are stuck to surfaces of the internal toothed plates and the external toothed plates which are not opposed to each other is also known.
However, in recent years, in order to reduce fuel consumption of a vehicle, it has been requested that the vehicle be made more compact and lighter, and an automatic transmission be made more compact and lighter to reserve the interior space of the vehicle, and also, an axial dimension of a wet multi-plate clutch used in the automatic transmission be made smaller.
In the conventional clutches, it is contemplated that a plate thickness of the core plate is decreased in order to reduce an axial dimension of a clutch pack; however, if the plate thickness of the core plate is decreased, heat resistance is reduced due to reduction of a heat absorbing ability of the metallic surface of the external toothed plate associated with the friction material, and, since the plate thickness is small, a face-pressure of the engagement surfaces between the internal toothed plate and the hub is increased and a face-pressure of the engagement surfaces between the external toothed plate and the clutch drum is increased, with the result that inconvenience such as indentation is generated at the engagement surfaces. Thus, the reduction of the plate thickness causes a problem.
Further, it has been requested that the axial length of the entire clutch be shortened, without reducing a heat dispersing amount of the external toothed plate to enhance the heat resistance.